How do I pay Apple Pay what I owe?

You can't owe Apple Pay anything. Its a payment conduit, not a credit card or loan.

Do you mean the Apple Pay Later feature where you take out a loan to buy stuff, and can pay it back in installments?

click here ➜ Make an Apple Pay Later loan payment - Apple Support

Do you mean Apple [credit] Card?

click here ➜ How to make Apple Card payments - Apple Support

You owe a payment method (credit, debit or prepaid debit card) not Apple Pay. Can you provide any more details? If not you can call Apple Support at 1-800-MY-APPLE.